in addition to; besides; as well as
Used to add another item or factor, often in formal or written contexts. Similar to に加え, but に加えて is slightly more common in spoken Japanese.
See also: に加え
日本語に加えて、英語も話せます。
In addition to Japanese, I can also speak English.
雨に加えて風も強くなってきた。
Besides the rain, the wind has also become stronger.
に加え and に加えて both mean 'in addition to', but に加え is slightly more formal/written while に加えて is more common in speech.
さらに is an adverb meaning 'furthermore' that can stand alone at the start of a sentence, while に加えて directly attaches after the noun it adds to.
そのうえ means 'moreover' and links two full clauses, whereas に加えて attaches to a noun phrase to add it to what precedes.
だけでなく means 'not only... but also' and is more colloquial, often used for contrast, while に加えて is neutral and simply adds an extra item.
From the verb 加える (くわえる, 'to add') in its te-form, used as a conjunction meaning 'adding to that'.